1. Field of Invention
This invention relates to testing of analog circuitry. The invention specifically relates to a pass-fail technique for verifying proper operation of an analog circuit in connection with a comparison-type digital circuit tester where the analog output signals of a known good reference circuit are compared in real-time with the corresponding analog output signals of an unknown circuit unit under test to generate a digital pass-fail signal.
Heretofore analog circuits have been tested by inspection of signal levels and waveforms through the use of an oscilloscope or through the use of a digital data base representing an analog waveform. The oscilloscope technique is relatively slow and requires the attention of an operator trained in the use of the oscilloscope in order to identify failures or defects in the circuit under test. Moreover, oscilloscope testing techniques are not easily adapted to high volume assembly line testing since real-time waveform observations are required which are subject to human evaluation and judgmental error.
With the development and relatively wide acceptance of digital circuit technology, there has also developed efficient and high-speed techniques for testing complex digital circuitry. The digital testing techniques have utilized the discrete state output signals inherent in digital circuitry as a basis for rendering judgments on the operation of the digital system under test. Complex and rapid testing techniques have been developed to trace circuit faults and to identify defective circuitry.
In digital systems, portions of the system may be subassemblies having analog signal outputs, such as analog signal generators, converters and power supplies. Heretofore automatic test equipment manufacturers have addressed the problem of checking such analog output signals by providing complex, expensive and sophisticated computer-based testers which digitize the analog output signals and then compare the digitized result with a digital data base. Such testing techniques are inherently limited by the size of the data base and speed of circuit response. What is needed is a testing technique which is easily adapted to comparison-type circuit testers particularly useful in a production environment. In particular what is needed is a technique for real-time testing whereby transient failures on an analog circuit can be traced automatically according to rules of logic.
2. Description of the Prior Art
Comparison-type digital logic test techniques are well-known, as for example Giedd, et al. U.S. Pat. No. 3,614,608, and Drabing, et al. U.S. Pat. No. 4,125,763. A comparison-type test system for testing points in a circuit using analog tape recording and storage techniques is disclosed in U.S. Pat. No. 3,522,532.